Binding screw having electrically insulated head

ABSTRACT

A binding screw having a threaded shank and an enlarged head coated with a plastic insulating material. The head includes a usual slot which is cut through the insulating material exposing the base metal.

United States Patent [72] Inventors Cuslaw Mncl lewlcz Trumbull; JamesE. Meehan. Bridgeport, both 01, Conn. 211 App]. No 121.939 [221 FiledMay 9, 1968 [45] Patented July [3, 1971 [73] Auignee Ihrvey l-lubbell,Incorporated lrldgepurt, Conn.

[54] BINDING SCREW HAVING ELECTRICALLY INSULATED HEAD 5 Claims, 6Drawing Flgz.

(52] U.S.CI. 85/1, lO/IO. l0/27, 85/9, 1I7/2l, 117/113, lI7/l28.4,174/54. 200/ I 53 [51] Int. Cl ..F16b 23/00, Fl 6b 35/00 [50) run-rm85/1C,

NH. 1 JP, 9, 1,54, 37,45; l5l/38,7; 10/10, 27 P, 27.5.3; 117/21, 3,116?, 128.4, I27, 132C, 23033,4; l74/l66, I93, 54, 58

[S6] Relerenees Clted UNITED STATES PATENTS 268,697 12/1882 Lewis .185/54 UX 1,717,388 6/1929 KeeIerW 10/10 2,145,744 1/1939 Whitneyum 485/9 UX 2,191,171 2/1940 Leeetal 174/166 2,326,455 8/1943 Gray .4 85/372,724,303 11/1955 l-lolcomb.... 85/10 2.844.489 7/1958 Gemmer 117/21 X2,981,651 4/1961 Arnold 117/21 X OTHER REFERENCES MATERIALS IN DESIGNENGINEERING, February, 1960, pp. 91- 95,copy available in I I7 21.

Primary Examiner- Ramon S. Britts Anorney- Wooster, Davis & CifelliABSTRACT: A binding acrew having a threaded shank and an enlarged headcoated with a plastic insulating material. The head includes a usualslot which is cut through the insulating material exposing the basemetal.

BINDING SCREW HAVING ELECTRICALLY INSULATED HEAD BACKGROUND OF THEINVENTION This invention relates to a binding screw and a method ofmaking the same and, more particularly, to such a screw having a layerof plastic insulation material deposited upon its head. This inventionfurther relates to the use of insulated head binding screws with groupedswitches to prevent arcing between adjacent screwheads.

It is common to group switches in an outlet box for convenientlyproviding a group of AC switches at a single location. However, whenhigh-voltage circuits, for example, 20 a. 277/480 v. AC, are controlledby the switches a potentially hazardous condition exists in the outletbox. The usu =.l laterally outwardly extending binding screws of each ofthe closely mounted switches present exposed live metal parts acrosswhich arcing may occur when a 700 volt potential exists between adjacentbinding screws. To prevent this condition, the National Electrical Codeprovides in part that: "Snap switches shall not be grouped or ganged inoutlet boxes unle s they can be so arranged that the voltage betweenexposed live metal parts of adjacent switches does not exceed 300 volts.

One solution to the problem of grouping AC switches in high voltagecircuits has been to eliminate the binding screws altogether and to backwire current-carrying leads to the switch bodies. The leads may becaptured by suitable internal spring clips in a known manner. However,these clips often lose their resilience and become unreliable.Therefore, it would be desirable to retain the positive mechanicalsecuring action of a binding screw of either a back-wired switch or abinding-head type while ensuring that no arcing will occur betweenadjacent switches when closely mounted in an outlet box.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of thisinvention to provide a binding screw with an electrically insulated headand a method ol'forming the same.

Another object of this invention is to provide a binding screw having alayer of plastic insulating material deposited upon its head and havingthe base metal material exposed in the slot.

A further object is to provide a plurality of grouped switches with thebinding head screw described in the above paragraphs.

To accomplish the primary object, in one form, there is provided abinding screw having a threaded shank with an enlarged electricallyinsulated slotted head at one end. The head includes a layer of plasticinsulation material surrounding the metal head of the screw and the slotextends through the layer of plastic insulation material and into themetal head to expose the base metal of the head.

BRIEF DESCRIPTION OF THE DRAWING Other objects and further details ofthat which is believed to be novel and the present invention will beclear from the following description and claims taken with theaccompanying drawing wherein:

FIG. 1 is a front elevational view of an outlet box with the coverremoved housing a series of grouped AC switches having binding headscrews constructed in accordance with this invention;

FIG. 2 is a perspective view of a screw blank with a coated head;

FIG. 3 is a perspective view of the screw blank of FIG. 2 in which ashank has been threaded;

FIG. 4 is a perspective view similar to that of FIGS. 2 and 3 in whichthe head has been slotted;

FIG. 5 is a side elevational view partly in section showing a bindinghead screw constructed in accordance with the invention;and

FIG. 6 is a schematic side elevational view of a holding rack of screwblanks positioned above a trough of coating material DESCRIPTION OF THEPREFERRED EMBODIMENT With particular reference to the drawing there isillustrated in FIG. I a plurality of AC switches 10 mounted in an outletbox 12 upon mounting lugs 14. Each switch includes a cover 16 made of aplastic insulating material, a toggle l8, and a plurality of laterallyoutwardly extending binding screws 20. A mounting strap 22 carryingsecuring screws 24 retains each switch in position in the outlet boxupon the mounting lugs. The lateral position of each binding screw 20 isdetermined by the size lead captured thereby when the lead is secured bythe head. Therefore, it should be noted that the heads of the bind ingscrews of adjacent switches may be extremely close together and in factmay be in direct contact. In order to satisfy the above-quotedrequirement of the National Electrical Code the binding screws 20 havebeen formed to eliminate the conventional exposed live metal parts ofthese screws.

The novel binding screw 20 of this invention is clearly illustrated inFIGS. 4 and 5. This screw includes a metallic threaded shank 26 havingan enlarged metallic head 28 at one end. A layer of synthetic plasticinsulating material 30, such as nylon, is bonded to the top and sides ofhead 28 and a slot 32 is formed through the plastic layer 30 into thehead 28 (see FIG. 5). The dielectric coating material has been selectedto be hard enough to allow it to be machined in the slotting operationwithout hurting while also being resilient enough to prevent chippingand cracking. The design of this novel binding screw 20 relies on adielectric coating thickness of at least 0.010 inch to provide adequateinsulation. It has been determined that when binding screws constructedin accordance with this invention are positioned with their headstouching one another (i.e. having a total insulation thickness of 0.02inch) they are able to withstand approximately 3,000 volts therebetweenwithout breakdown of the dielectric. Therefore, it should be apparentthat their use in 277/480 v. AC circuits is well within acceptablelimits. It should also be pointed out that although these binding screws20 have been developed to solve a particular problem-viz grouped ACswitchesthey will find usage in a great number of applications whereexposed live binding screws are present.

Although the screwheads 28 may be coated after slotting, this has beenfound to be disadvantageous for several reasons. First, the screwdriverwill often cut through the insulation in the slot and tear out sectionsof insulation in the slot and on the top surface of the head. Second,the coated slot generally does not have flat walls to transmit appliedtorque from a screwdriver which has a tendency to slip out of the slot.Third, the resilient insulating material in the slot presents a plasticcushion between the screw and the screwdriver which results in anuncertain feeling when tightening the screw. Fourth, the slot does notcoat uniformly giving a poor appearance to the finished article.Finally, there is no visual means of inspecting for adequate coatthickness. The novel binding screws 20 illustrated in FIGS. 4 and 5overcome the above disadvantages with the base metal material exposed inthe slot thus eliminating the false sense of security which might beassociated with a totally coated head.

The binding screws 20 of this invention are formed by heading a usualscrew blank 34, loading a plurality of blanks 34 in a holding rack 36(note FIG. 6), heating the screw blanks and immersing the heads of theblanks into a trough 28 of suitable insulating material 40 which may bein the form of an airagitated fluidized bed of nylon powder or aliquefied plastic. Holding rack 36 is moved downwardly to allow only thehead of the blank to be immersed in the insulating material. Theimmersed heated screwheads melt the nylon particles or react with theliquefied plastic so that a layer of plastic insulation is firmlyadhered to and is built up upon the screw blanks. The thickness of thelayer is controlled by the temperature of the blanks as well as thelength of immersion time. The coated screw blank 42 (note FIG. 2) isremoved from the holding rack 36 and may be transferred to a threadingapparatus where a thread 26 is rolled upon its shank in a known manner.it then may be slotted as at 22 to form the finished article (note H68.4 and 5).

It is believed that the many advantages of this invention will now beapparent to those skilled in the art. As set forth in the objects abovethere is provided an inexpensive binding screw which may be used inhigh-voltage circuit AC switches, particularly those grouped in a commonoutlet box to prevent arcing between binding head screws of adjacentswitches. Of course, the binding screw will find numerous applicationsin other environments.

It should be understood that the present disclosure has been made onlyby way of example and that numerous changes in details of constructionand the combination and arrangement of parts may be resorted to withoutdeparting from the true spirit and scope of the invention as hereinafterclaimed.

What we claim is:

l. A binding screw comprising: a threaded metallic shank; an enlargedmetallic head integrally formed with said shank at one end thereof, saidhead having a top and sides; a relatively thin layer of plasticelectrical insulation material secured to and surrounding the top andsides of said head; and a transverse slot extending through said layerof plastic insulation material into said head, thereby exposing the basemetal of said metallic head throughout the sidewalls of said slot.

2. The binding screw defined in claim I wherein said layer of plasticinsulation material is bonded to said head.

3. A method of forming a binding screw comprising the steps of: forminga head screw blank; heating the blank; immersing the heated head in amass of plastic insulation material for a predetermined time therebybonding the insulation material to the head and building up aninsulation layer of desired thickness; removing the coated blank fromthe mass of insulation material; threading the shank of the screw blank;and slotting the head of the screw blank through the insulation layerand into the metal head.

4. The method of forming a binding screw as defined in claim 3 whereinsaid mass of plastic insulation is a fluidized bed of nylon particlesformed by continuously agitating the nylon particles with an airblastpassing therethrough.

5. The method of forming a binding screw as defined in claim 3 whereinsaid mass of plastic insulation is a bath of liquid plastic material.

1. A binding screw comprising: a threaded metallic shank; an enlargedmetallic head integrally formed with said shank at one end thereof, saidhead having a top and sides; a relatively thin layer of plasticelectrical insulation material secured to and surrounding the top andsides of said head; and a transverse slot extending through said layerof plastic insulation material into said head, thereby exposing the basemetal of said metallic head throughout the sidewalls of said slot. 2.The binding screw defined in claim 1 wherein said layer of plasticinsulation material is bonded to said head.
 3. A method of forming abinding screw comprising the steps of: forming a head screw blank;heating the blank; immersing the heated head in a mass of plasticinsulation material for a predetermined time thereby bonding theinsulation material to the head and building up an insulation layer ofdesired thickness; removing the coated blank from the mass of insulationmaterial; threading the shank of the screw blank; and slotting the headof the screw blank through the insulation layer and into the metal head.4. The method of forming a binding screw as defined in claim 3 whereinsaid mass of plastic insulation is a fluidized bed of nylon particlesformed by continuously agitating the nylon particles with an airblastpassing therethrough.
 5. The method of forming a binding screw asdefined in claim 3 wherein said mass of plastic insulation is a bath ofliquid plastic material.